Method of producing light weight plasters



Patented Feb. 25, 1936' r METHOD OF PRODUCING LIGHT WEIGHT PLASTERS John0. Best and Frank Lee Marsh, Medicine Lodge, Kans., assignors to TheBest Brothers Keenes Cement Company, Medicine Lodge, Kans., acorporation of Kansas No Drawing. Application September 18, 1933, SerialNo. 689,957

6 Claims. (Cl. 106-34) Our invention relates to a method of producingfor this purpose, but such chemicals have the light weight plastershaving advantages over disadvantage that they remain in the mixedmaplasters now in use, and better adapting the material after it issubsequently calcined, and thus terial for use as a wall plaster, forcasting in serve as an accelerator when the calcined matevarious typesof molds, for surgical bandages, as rial is again used for plaster.

an insulator against sound, heat or cold and as We have discovered,however, that by reca1- an acoustical material. cining the plaster ofordinary fine grade produced In the manufacture of plaster of thischaracaccording to the ordinary methods, such as that ter, the dihydrategypsum rock crushed or ground described above, after the material hasbeen to desired fineness is calcined at a temperature mixed through theset at a temperature and 10 suitable for the production of the grade ofplaster rate of through-put to again form a hemihydrate desired, cooledand re ground to proper particle We are able to produce a still finermaterial which size for ultimate use, and an agent for acceleratcan becast or molded to produce articles of exing set of the plaster added ifdesired, all in actremely light weight having the appearance of cordancewith ordinary practice. a solid object, due to comparatively small voidsf It is known that a material of fine particle size between theparticles. 0 can be obtained by calcining gypsum rock at a We have alsodiscovered that an acid, that will temperature and rate of through-putof the kiln be decomposed by subsequent calcination, when to convert thedihydrate to a hemihydrate, mix employed as an accelerator, will belargely taken ing the hemihydrate with water in considerable up byneutralization with any alkali in the gypsum 20 excess over the amountneeded to attain normal or by chemical combination with any carbonateconsistency and agitating the mixture during therein to form a calciumsalt, and that any small what would ordinarily be the setting perioduntil residue of such acid which may be left in the mixthe plaster hasreverted to .dihydrate of gypsum ture is driven off in subsequentrecalcination of in very fine crystalline form. the product. We havefound that sulphuric acid For example, if a plaster is originally groundis particularly adapted for use as such an accelerto a fineness at whichapproximately 80% of the ator and prefer to use such acid in thepractice material passes through a 100-mesh screen, and of our process.has a water ratio at normal consistency of ap- It is, therefore, theobject of our invention to proximately 50 to 60 cubic centimeters ofwater provide the method herein described for producto 100 grams ofplaster, when it is agitated tion of finer or lighter plaster which,when cast through what would ordinarily be the setting or molded, usedon a wall, or for other purposes, period, the resulting dihydrate willbe of a finewill have greater bulk but lighter weight than ness to passup to a 500-mesh screen, and will plasters produced according toordinary methods.

require as high as 800 cubic centimeters of water In practicing ourmethod we reduce the di- 35 to 100 grams of the plaster to attain normalconhydrate gypsum rock to proper particle size for sistency.calcination, calcine the reduced rockat a tem- Mixing the plasterthrough the set has been perature and rate of through-put to convert thefound to be particularly advantageous with dihydrate to the hemihydratesuitable for use as a plasters having fairly short setting times, sayplaster, mix the calcined product through what 40 from 15 minutes down,and it is customary to add would ordinarily be the setting period withwater the water to the plaster gradually as the mixture in excess ofthat required to attain normal conbegins to stiffen under the process ofagitation, sistency to reconvert the product to the dihydrate because,if the setting time of the plaster is too and preferably adding anaccelerator, such as sulslow or too much water is added in thebeginning, phuric acid, for hastening the setting time of the 45 thematerial will either not recrystallize in the ultimate product. lightWeight form or the production of light After the material has revertedto a very fine weight material will be only partial. crystalline form ofthe dihydrate by continuous By adding an accelerator of some kind to theagitation through the setting period, we then replaster, a wider rangeof original setting time in calcine the finely recrystallized gypsumproduct the plaster is permissible, and the full amount at a temperatureordinarily employed for the of the mixing water can generally be addedat manufacture of white plaster, to thereby again the beginning of theprocess, thus simplifying form thehemihydrate and produce aplaster prod-1 it considerably. It has been customary to use uct lighter, fiuffierand bulkier than that heretosome form of aluminum sulphate or other alumfore produced by ordinary methods. 55

Cast or molded articles made from plaster produced from our improvedmethod will be bulkier and lighter in weight than those produced fromordinary methods, and, while not having the strength of objects madefrom ordinary plaster, have been found to have a tensile strength ofapproximately 125 pounds to the square inch, the finished product havinga solid appearance, due to the fineness of the actual particles and ofthe pores formed by drying of the finished article.

What we claim and desire to secure by Letters Patent 1's:

1. The method of producing light weight plaster including calcininggypsum at a temperature to produce a hemihydrate plaster, mixing theplaster with water, agitating the mixture until the plaster hasrecrystallized as a dihydrate in finer particle size, and calcining thedihydrate at a temperature to produce a hemihydrate.

2. The method of producing light weight plaster including producing ahemihydrate plaster by calcining gypsum, mixing the plaster with Waterin excess of that required for hydration, agitating the mixture untilthe plaster has reverted to dihydrate of gypsum in fine crystallineform, and recalcining the plaster at a temperature under that requiredfor rendering the final product anhydrous.

3. The method of producing light weight plaster including calciningdihydrate gypsum at a temperature of approximately 360 F. to form ahemihydrate plaster, mixing the plaster with water, agitating themixture until the plaster has reverted to dihydrate of finer particlesize than that of the original product, and recalcining the dihydrate atapproximately 360 F. to produce a hemihydrate.

4. The method of producing light weight plaster including calciningcrushed or ground gypsum rock, mixing the calcined material with water,adding an accelerating agent to the mixture, agitating the wet mixtureuntil it has recrystallized in a finer particle size, and recalciningthe material.

5. The method of producing light weight plaster including calciningcrushed or ground gypsum rock, mixing the calcined material with water,adding to the mixture an accelerating agent that will decompose orbecome volatilized during recalcination of the mixture, agitating thewet mixture until it has recrystallized in a finer particle size, andrecalcining the material.

6. The method of producing light weight plaster including calciningcrushed or ground gypsum rock, mixing the calcined material with water,adding sulphuric acid to the mixture as an accelerating agent, agitatingthe wet mixture until it has recrystallized in a finer particle size,and recalcining the material.

JOHN C. BEST. FRANK LEE MARSH.

DISCLAiMER 2,031,585.J0hn 0. Best and Frank Lee Marsh, Medicine Lodge,Kans. METHOD OF PRODUCING LIGHT WEIGHT PLAsTE'Rs. Patent dated February25, 1936. Disclaimer filed February 21, 1939, by the assignee, NationalGypsum Company. I Hereby enters this disclaimer to claims 2, 4, and 5 insaid specification.

[Oflicz'al Gazette lblarch 21, 1939.]

